L.H. TAGLE*, F.R. DIAZ and A. OPAZO

Facultad de Química
Pontificia Universidad Católica de Chile
P.O. Box 306, Santiago, CHILE
e-mail: ltagle @ puc.cl

(Received: March 3, 2001 - Accepted: June 6, 2001)

RESUMEN

Las propiedades térmicas de poli(ésteres) conteniendo silicio o germanio en la cadena principal y derivados de los difenoles bis(4-hidroxifenil)-difenilsilano o bis(4-hidroxifenil)-difenilgermano y los cloruros de los ácidos isoftálico y tereftálico fueron estudiadas mediante calorimetría diferencial de barrido y termogravimetría dinámica. Los poli(ésters) derivados del ácido tereftálico mostraron mayores valores de Tg y estabilidad térmica que aquellos derivados del ácido isoftálico. Los poli(ésteres) conteniendo Ge mostraron mayores valores de Tg pero menores valores de temperatura de descomposición térmica que sus análogos conteniendo Si.

PALABRAS CLAVES: Poliésteres, germanio, silicio, estabilidad térmica, temperatura de transición vítrea.

ABSTRACT

The thermal properties of poly(esters) containing silicon or germanium in the main chain and derived from the diphenols bis(4-hydroxyphenyl)-diphenylsilane and bis(4-hydroxyphenyl)-diphenylgermane and isophthaloyl or terephthaloyl acid dichlorides were studied by differential scanning calorimetry and dynamic thermogravimetry. Poly(esters) derived from terephthalic acid showed higher Tg and thermal stability values than those derived from isophthalic acid. Poly(esters) containing Ge showed higher Tg values but lower thermal decomposition temperatures when compared with the analogous with Si.

KEYWORDS: Poly(esters), germanium, silicon, thermal stability, glass transition temperature.

INTRODUCTION

Dynamic thermogravimetric analysis has been widely used as a tool of the investigation of the thermal stability of polymers, and the thermograms provide information about the sample composition, thermal stability and the kinetics data relating to the chemical changes which occur on heating. The thermal degradation of polymers under normal conditions of use is the principal factor limiting the applications of these materials, and important properties are affected adversely as the degradation proceeds [1-3].

Silicon-containing polymers have been described for many years and their properties studied [4], but polymers containing germanium in the main chain and bonded to four carbon atoms have not been described, with the exception of poly(ferrocenylgermanes) [5] and poly(diphenylacetylenes) [6] in which the Ge atom is in the side chain. Poly(germanes) with a general formula (GeR₂)_n have inorganic nature and have been synthesized according electrochemical [7] or chemical methods [8]. In this sense we have described the synthesis of poly(carbonates) and poly(thiocarbonates) [9], poly(esters) [10] and poly(amides) [11] derived from monomers containing Si or Ge in the main chain and their properties studied.

In this work we studied the thermal properties of poly(esters) derived from diphenols containing Si or Ge in the main chain. The glass transition temperatures and the thermal stability were studied by DSC and thermogravimetry, respectively, and the kinetics parameters associated with the thermal process determined. The results are discussed as a function of the nature of the heteroatom, Si or Ge, and the nature of the diacid, isophthalic or terephthalic acid.

EXPERIMENTAL

Poly(esters) were synthesized from the diphenols bis(4-hydroxyphenyl)-diphenylsilane and bis(4-hydroxyphenyl)-diphenylgermane and isophthaloyl (m) or terephthaloyl (p) acid dichlorides under phase transfer conditions according to a procedure described earlier [10].

The Tg values were obtained using a Mettler Toledo DSC 821 calorimetric system. Thermogravimetric analyses were carried out in a Mettler TA-3000 calorimetric system equiped with a TC-10A processor, and a TG-50 thermobalance with a Mettler MT5 microbalance. Samples 6-10 mg were placed in a platinum sample holder and the thermogravimetric measurements were carried out between 30 and 800°C with a heating rate of 20°C min^-1 under N₂ flow.

RESULTS AND DISCUSSION

Poly(esters) with the following structures were synthesized under phase transfer conditions using several phase transfer catalysts in CHCl₂-CHCl₂ as solvent at 20°C according to a procedure described previously [10]. Poly(esters) were characterized by IR and ¹H and ¹³C NMR, and the structures were in agreement with those proposed.

Table I shows the Tg values for the poly(esters). Higher Tg values were obtained for poly(esters) derived from terephthalic acid which has a more symmetrical structure. The same behaviour has been described for poly(amides) derived of these diacids and diamines containing Si or Ge [11] and poly(carbonates) and poly(thiocarbonates) derived from diphenols containing Si or Ge [12]. On the other hand, poly(esters) containing Si showed higher Tg values than those containing Ge, which can be attributed to the larger size of Ge compared with Si, which implies larger bond lengths in poly(esters) derived from diphenols containing Ge, and consequently lower rotational barriers and lower glass transition temperatures [4]. The larger size of Ge can also increase the flexibility of the polymeric chain and decreases the corresponding Tg values.

Table I. Glass transition temperatures of the poly(esters)

The thermal stability of the poly(esters) was analyzed by dynamic thermogravimetry. Figures 1 ­ 2 show the thermogravimetric curves and Table II the thermal decomposition temperatures (TDT), taken as the temperature at 10% weight loss. Poly(esters) containing Si derived from terephthalic acid showed higher TDT values than those derived from isophthalic acid, which can be attributed to the more symmetrical structure of the first diacid. Poly(esters) containing Ge showed the same TDT value for both acids due probably that the size of the heteroatom has more influence than the nature of the diacid.

Fig.1. Thermogravimetric curves for poly(esters) Si-m and Ge-m

Fig.2. Thermogravimetric curves for poly(esters) Si-p and Ge-p

Table II. Thermal decomposition temperatures (TDT) and Kinetics parameters of the thermal decomposition of Poly(esters)

Poly(esters) containing Ge as heteroatom were more stable than those containing Si. The relationship between electronegativity, bond polarity and thermal stability of heteroatom polymers has been described [13]. In fact, due to the different electronegativity of the elements in polymers with heteroatoms as Si or Ge, the bond polarity can result in reduced thermal stability. In this case, the bond Si ­ C, due to the difference of electronegativity, has a slightly higher polarity than the Ge ­ C bond. Also the Ge ­ C bond is stronger than Si ­ C because the former has a higher bond enthalpy [14]. Both situations would explain that polymers with the same structure with Ge have higher thermal stability than those with Si. A similar behaviour has been described for heteropolymers as poly(amides) [11] and poly(carbonates) and poly(thiocarbonates) [12] containing Si or Ge in the main chain.

The kinetic parameters for the thermogravimetric weight loss were calculated according to the multiple linear regression method, using the kinetic equation:

where a is the fraction of the sample weight at time t, and k_n the specific rate with kinetic reaction order n. The reaction rates -(da / dt) were calculated using a differential technique with the heating rate (20°C min^-1) incorporated directly into the temperature versus sample weight-fraction data, according to the procedure developed by Wen and Lin [15]. The specific rates were calculated from the Arrhenius relation

where E is the activation energy, A the pre-exponential factor, T the absolute temperature, and R the gas constant. Equations (1) and (2) were combined and used in logarithmic form.

A computer linear multiple-regression program was developed to calculate the kinetic parameters E and A from linear least-squares fit of the data in a semilogarithmic plot of b versus 1/T. Figures 3 - 6 show those graphics and the results are summarized in Table II. The linearity of the plots was >0.995, although some scatter was detected at the beginning and at the end of each decomposition, which can be attributed to the difficulty of making accurate measurements at these points of an experiment, as is frequently encountered in kinetic measurements.

Fig.3. Arrhenius plot for the degradation of poly(ester) Si-m

Fig.4. Arrhenius plot for the degradation of poly(ester) Ge-m

Fig.5. Arrhenius plot for the degradation of poly(ester) Si-p

Fig.6. Arrhenius plot for the degradation of poly(ester) Ge-p

All polymers degrade in a single one stage and the best fits were obtained for n = 0.25. If a first or zero-order reaction are assumed, a non-linear plot is obtained. In this work the reaction order for the degradation process was obtained when the linearity over the entire decomposition range was achieved, and it is probably that this reaction order imply a very complex degradation process, composed by several superimposed degradation mechanism than can not be separated in a clear range of temperatures, that depend on the functional groups, the presence and nature of the heteroatom, and the phenyl side groups.

The activation energy was calculated from Eq. (3) being its value constant over the range considered. Poly(esters) containing Ge showed higher activation energy values which can be due to that the Ge-C bond is a stronger bond respect to the Si-C one [14].

The thermal degradation process in a solid state for condensation polymers is a very complex process and also complex reactions can occur, and consequently, the kinetic parameters have limited significance and it is difficult to suppose a constancy of them over the temperature range [16]. On the other hand, the solid state of the sample is far from the ideal and can change in the course of the degradation process. In spite of the above considerations, the thermal degradation is a very important tool to know the influence of the polymer structure on the thermal stability, and also to know the temperature at which the polymers can be used, as well as the activation energy associated with the total degradation process.

ACKNOWLEDGEMENTS

The authors acknowledge the financial support of FONDECYT through grant 8970011.

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